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Determinism Across Environments

When to Use

Use this when screenshots pass locally but fail in CI, or when setting up a project to avoid local-vs-CI drift from the start.

The Core Rule

Playwright's own docs: "Browser rendering can vary based on the host OS, version, settings, hardware, power source, headless mode, and other factors. Run tests in the same environment where the baselines were generated."

Decision

Scenario Right approach
New project Capture baselines inside Docker from day one
macOS dev, Linux CI Run --update-snapshots inside the Docker container
Which platform is "truth"? linux — CI runs there; *-linux.png are the canonical baselines

Pattern

Official Docker image

docker pull mcr.microsoft.com/playwright:v1.59.1-noble

Tag taxonomy: - v1.X.Y-noble — Ubuntu 24.04 LTS (current default) - v1.X.Y-jammy — Ubuntu 22.04 LTS

Recommended flags: --init, --ipc=host

Pin everything

"devDependencies": { "@playwright/test": "1.59.1" }

Pin the Docker tag to the same version. Never use :latest.

Capture baselines inside Docker

docker run --rm \
  --ipc=host \
  -v $(pwd):/work -w /work \
  --add-host=mysite.ddev.site:host-gateway \
  mcr.microsoft.com/playwright:v1.59.1-noble \
  npx playwright test --update-snapshots

Common Mistakes

  • Wrong: mcr.microsoft.com/playwright:latest → Right: pin the tag; :latest changes with every release
  • Wrong: Capturing on macOS, comparing in CI → Right: antialiasing differs; always use the same Docker image
  • Wrong: Custom Dockerfile without --with-deps → Right: missing fonts and system libs produce rendering differences

See Also